PHOENIX — Giants left-hander Barry Zito is shedding plenty of baggage while getting off to the best start of his career. But there’s one hefty duffel still slung over his shoulder — and it’s bright green-and-gold.

After leaving the A’s as a free agent three years ago, Zito is 0-3 with a 9.22 ERA against his former team. He’ll get another crack at them today when the Giants open their annual cross-Bay series at the Coliseum.

“I like the ballpark. I want to go back there and pitch well, sure,” Zito said. “But it’s nothing new, really. It’s been three years. It’s hard to feel I’m still connected.”

Especially because Zito hasn’t forgotten the way A’s fans treated him when he went back for the first time as a Giant in 2007.

“They booed the (heck) out of me,” Zito said. “It was pretty jarring. I wasn’t expecting them to cheer for me, but really? The things they were yelling “… it was awful. So yeah, bring it on. I hope we go out and beat those guys.”

Among current A’s, only Eric Chavez and Mark Ellis played with Zito. But the ballpark still holds plenty of memories for him. He has made 111 career starts at Oakland Coliseum — twice as many as the 55 he’s made at AT&T Park.

Giants manager Bruce Bochy said he hasn’t made a decision on No. 5 starter Todd Wellemeyer, but the homer-prone right-hander is almost certain to be skipped the next time through.

If the Giants decide to replace Wellemeyer when they need a No. 5 starter on May 29 vs. Arizona, they can pick from Madison Bumgarner, Eric Hacker, Joe Martinez and Kevin Pucetas.

There is an explanation for Nate Schierholtz’s .171 average over his past 12 games. He jammed his shoulder while diving in right field two weeks ago, and the soreness kept getting worse as he played through it, Bochy said.

Edgar Renteria (groin) and Mark DeRosa (wrist) took early batting practice on the field. Renteria is 100 percent and will be activated when eligible on Saturday, Bochy said.

Otto Warmbier was arrested in January 2016 at the end of a brief tourist visit to North Korea. He had been medically evacuated and was being treated at the University of Cincinnati Medical Center when he died at age 22.